The British and Irish prime ministers, Tony Blair and Bertie Ahern, issued an ultimatum Thursday to Northern Ireland's divided politicians: Elect a power-sharing administration by November — or your legislature will be disbanded.

Their declaration followed 3 1/2 years of diplomacy that has failed to revive a Catholic-Protestant administration, the intended centerpiece of the Good Friday peace accord that both prime ministers oversaw eight years ago. A previous coalition collapsed in October 2002 over an Irish Republican Army spying scandal.

Blair and Ahern stood side by side inside Navan Fort, the seat of power of the ancient kings of Ulster, to announce that the Northern Ireland Assembly would reconvene May 15 for the first time since that 2002 crisis.

The end result, they said, must be a vote by members to form an administration led by the province's extremes of opinion: Ian Paisley's Democratic Unionists, who represent most of the British Protestant majority, and Gerry Adams' Sinn Fein, the IRA-linked party that represents most Roman Catholics.

Blair said he and Ahern devoted years to narrowing the ground between the two foes — and the time had come for "the ultimate decision." He said Nov. 24 represented the moment when "we close the chapter or close the book."

"We have reached a point in the process where the parties must decide," agreed Ahern. "We are giving them a reasonable but finite time to do so."

If local power-sharing wasn't possible, Blair said, the only alternative was for the British and Irish governments to exercise "a necessarily more rigid will imposed from outside" on Northern Ireland — an obvious warning to Protestants, who long have criticized Irish government involvement in the province's affairs.

But Paisley, who celebrated his 80th birthday Thursday, dismissed the Nov. 24 deadline as irrational bullying. He said his party would form a Cabinet with Sinn Fein only if Adams' party accepts the legitimacy of the Northern Ireland police force and if the IRA disbands.

He accused the IRA of lying about its landmark commitments last year to renounce violence and disarm, citing Tuesday's shotgun slaying of Denis Donaldson, a former Sinn Fein official who had been exposed as a British spy. While the IRA denies involvement, police say IRA members are suspects.

Paisley said the Democratic Unionists "will in no circumstances be in the business of putting terrorists and criminals into the government of Northern Ireland. Entrance to government cannot be dependent on a date, but only when terror and crime carried out by those allied to a political party is gone forever."

Sinn Fein offered a guarded welcome laced with skepticism.

Adams pressed the British and Irish governments to spell out "the new joint government arrangements and the accelerated all-Ireland cooperation and action that will replace the Assembly if the DUP (Democratic Unionist Party) is not prepared to share power."

Public opinion in Northern Ireland has grown increasingly cynical about their politicians, particularly the Assembly's 108 members, who have kept drawing annual salaries and expenses worth $150,000 a year despite having no jobs to do. Britain has sustained the system on financial life support to keep its members on standby for a successful power-sharing vote.

"The people of Northern Ireland would find it absurd just to carry on with this Assembly — and nobody actually doing anything," Blair said, adding, "We can't let this go on forever."

Blair and Ahern, who both rose to power in 1997 and made peace in Northern Ireland a shared high priority, both say they consider the Good Friday pact the greatest accomplishment of their careers — a legacy they concede will be incomplete if power-sharing fails to take root.

Both men could find themselves out of office within the next year: Blair is facing growing pressure within his Labour Party to step down as prime minister well in advance of the next British general election expected in 2009, while Ahern's Fianna Fail party is languishing at a historic ebb in opinion polls with a year to go before Ireland's next vote.

Sinn Fein, which hopes next year to ride a tide of anti-establishment feeling to strong gains in the Republic of Ireland elections, is seeking to help run coalition governments in both parts of Ireland — a unique position to push the party's aim of politically unifying the island.